Notes-Part-1-Class-12-Biology-Chapter-1-Reproduction in Lower & Higher Plants-Maharashtra Board

Modes of Asexual Reproduction :

1-Fragmentation : Fragmentation: It is a type of asexual reproduction observed in lower plants, e.g. algae.

Multicellular organisms break into small pieces called fragments which develop into new plant.

These fragments are formed due to different reasons like accidental breakdown, death and decay of cells, etc.

2-Budding : Budding in plants is an artificial method of propagation in which a single bud is joined or grafted on the stock plant. It is a type of asexual reproduction.

• It is of very common occurrence in unicellular organism yeast.
• It is observed in favourable condition.
• Mother cell produces small outgrowth which is known as bud.
• Buds may be one or more and on separation they grow as new individual.

3-Spore formation : It is of very common occurrence in lower plants.

• It occurs by production of motile zoospores that are formed in sporangia.
• Flagellated zoospores when liberated can grow independently into new individuals.
• Biflagellate zoospores are formed in algae Chlamydomonas.

4-Fission

• Binary Fission
• E.g. Amoeba, Paramoecium

5-Conidia formation

• E.g. Fungus Penicillium

6-Gemmule formation

• E.g. Sponges

Artificial methods of vegetative reproduction :

In artificial method cutting and grafting are two methods used to propagate desired varieties of plants-

(1) Cutting : small pieces of plant parts having one or more buds are selected for propagation, e.g, Stem cutting - Rose, Root, Cutting—Blackberry and Leaf cutting- Sansevieria

• Small piece of plant part selected.
• Must possess one or more bud.

• Stem cutting-e.g. Rose, Bougainvillea
• Leaf cutting-e.g. Sansevieria
• Root cutting-e.g. Blackberry

(2) Grafting

• Joining of two plant parts stock and scion
• Rooted plant — Stock
• Joined plant — Scion
• Grow together as one plant

• Stem grafting-e.g. Apple
• Bud grafting, budding-e.g. Rose

(3) Tissue Culture

• A small amount of plant tissue is carefully and aseptically grown to get plantlets.
• Micropropagation — Modern method to get plants from tissue culture. e.g. Orchids

Flowers : Flower is a specialized reproductive structure which produces haploid gametes and ensures that act of fertilization will take place.

1) Accessory whorls:

• Calyx made up of sepals
• Corolla made up of petals

2) Essential whorls :

• Androecium Stamen-> Filament-> Connective->Anther lobe—Pollen grains
• Gynoecium Pistil -> Style ->Stigma -> Ovary— ovule

Sexual reproduction - Two major events :

Meiosis : Production of gametes (n)

• Male gametes in anther
• Female gametes in ovule

Fusion of gametes (Fertilization) ->Diploid zygote -> Embryo -> New plant—(2n) sporophyte

Diploid sporophyte is dominant plant body —> Meiosis -> Haploid spores-> gametophyte->  Reduced structure -> Wlthin flowe-> garnetes

T. S. of anther (Transverse section of anther) :

Internally it shows four chambers called microsporangia or pollen sacs.

The anther consists of two main parts, viz., anther wall and microsporangium or pollen sac. The wall of the anther can be differentiated into four layers. viz., epidermis, endothecium, middle layers and tapetum.

• The epidermis is the outermost layer of the anther wall. It is made up of flattened cells which are protective in function.
• The endothecium lies internal to the epidermis. It is made up of a single layer of cells. The cells of endothecium show fibrous thickenings on radial walls.
• Internal to the endothecium, lie 1 to 3 layers of parenchymatous cells forming middle layers of the anther wall. The cells of middle layers degenerate at maturity during the formation of microspores.
• The tapetum is the innermost nutritive layer of the wall of the anther, consisting of a single layer of cell surrounding the sporogenous tissue.

• Microsporangium contains microspore mother cells (2n) which undergo meiosis to form microspore tetrad.
• Haploid (n) microspores separate from tetrad.
• When microspores develop wall around it then it is known as pollen grain.
• From archesporial cell primary parietal cell forms anther wall and sporogenous cell forms sporogenous tissue.

Structure of microspore : Typical pollen grain is a non-motile, haploid, unicellular body with single nucleus.

It is surrounded by a two layered wall called sporoderm.

Exine (outer layer) :

• Thick
• Made up of complex, non-biodegradable, substance called sporopollenin.
• Smooth or with a sculptured pattern.
• Resistant to chemicals.
• At some places exine is very thin showing thin areas known as germ-pores.
• These are meant for the growth of emerging pollen tube during germination of pollen grain.

Intine (inner wall layer): The inner wall layer, intine consists of cellulose and pectin. Forms pollen Tube smooth.

Development of male gametophyte :. It is considerably reduced. Develops in flower.

First mitotic division —> Two unequal cells ->(i) Vegetative cell (ii)Generative cell. Second mitotic division -> In generative cell ->equal cells.

• Microspore or pollen grain is first cell of male gametophyte.
• The protoplast of pollen grain divides mitotically to form two unequal cells — a small thin walled generative cell and a large naked vegetative or tube cell.
• The generative cell possesses thin cytoplasm and a nucleus. It separates and floats in the cytoplasm of vegetative cell.
• The vegetative, possesses thick cytoplasm, irregular shaped nucleus and the reserved food.
• In majority of the angiosperms, the pollen grains are liberated at two-celled stage after the dehiscence of the anther.
• The generative cell of the pollen grain divides by mitosis to form two male non-motile gametes. It is either occur in pollen grain or in pollen tube.
• Pollen grains are shed in either two celled stage or three celled stage. Pollen grains are lodged on stigma of pistil.

Parts of ovule :

• Funiculus : The stalk of the ovule is called the funicle or funiculus. The funicle attaches the ovule with the placenta.

• Hilum : The point at which the funicle is attached to the body of the ovule is called hilum..
• Anatropous ovule : CllI'V€d ovule, where micropyle is near stalk.
• Nucellus : It is made up of diploid parenchymatous cells.
• Integuments : The protective coverings of the nucellus are called integuments. Outer and inner are two protective coverings.
• Micropyle : Narrow opening at apex. The integuments do not completely cover the nucellus. They leave a small opening called micropyle at the tip.
• Chalaza : The basal part of the nucellus is called chalaza..
• Female gametophyte or embryo sac : In a mature ovule, the nucellus shows an oval-shaped structure towards its micropylar end called embryo sac or female gametophyte It remains embedded in nucellus. It is oval, elongated, multicellular seven celled structure.

Development of female gametophyte :

• The diploid megaspore mother cell undergoes meiosis to form a linear tetrad consisting of four-haploid megaspores.

Generally, the chalazal megaspore becomes the functional megaspore. The other three megaspores degenerate.

• The chalazal megaspore (fertile megaspore)is the first cell of the female gametophyte. It undergoes enlargement and develops into the female gametophyte.
• The haploid nucellus of chalazal megaspore undergoes three successive free nuclear mitotic divisions to produce eight nuclei. Of these, the first mitotic division results in the formation of two nuclei.
• Both these nuclei undergo two successive mitotic divisions resulting in the formation of four nuclei at both the poles.

In the meantime, one nucleus from each pole called polar nucleus moves towards the centre of the embryo sac and fuse to form a diploid nucleus called secondary nucleus.

• The three nuclei at the micropylar end are organised to form a three-celled structure called egg apparatus, while the other three nuclei at the chalazal end reorganise to form three antipodal cells.
• The egg apparatus consists of a central cell called egg cell or female gamete which is flanked by two lateral cells called synergids.

The female gametophyte consists of an egg apparatus, a secondary nucleus and three antipodal cells, A seven celled 8 nucleated structure.